Previous studies with small cell lung cancer (SCLC) cell lines have demonstrated that gastrin-releasing peptide (GRP), the mammalian counterpart to bombesin, is expressed in a significant fraction of SCLC lines. To assess its role in causing autocrine stimulation, initial experiments attempted to demonstrate specific binding to SCLC membrane fractions, with little success. Accordingly, efforts to define rapid physiologic responses to GRP were undertaken. These studies demonstrated clearly that in 5/11 SCLC cell lines tested there was evidence of calcium mobilization following GRP or bombesin stimulation. The structure-activity relationship of this effect was in accord with that expected for GRP receptors in gut, brain, and anterior pituitary cells. Further studies focused on the relationship of this response to inositol phosphate metabolism. In a SCLC cell line with a brisk response to GRP with increased intracellular calcium, there was evidence of increased inositol 1,4,5 trisphosphate within seconds of addition of bombesin congeners. Both the mobilization of calcium and inositol phosphate turnover were inhibited by cholera toxin and active phorbol esters. In the presence of pertussis toxin there was also a less complete stimulation of inositol phosphate turnover. In corollary studies it was demonstrated that the cell lines with the best response to bombesin showed constitutive expression of L-myc and prepro GRP without expression of c- or N-myc. In contrast, cell lines without evidence of response to bombesin had constitutive N- or C-myc expression. These studies suggest that an order of progressively malignant and distinct phenotypes can be defined in SCLC cell lines. A most "differentiated" cell line would be those which produce and respond to GRP, and also produce L-myc. A less differentiated cell line would neither produce nor respond to GRP and express abundant c- or N-myc. GRP (+), L-myc (+), c-myc (-), and N- myc (-) cell lines would represent those in which an effort to block a potential autocrine loop involving GRP might be most successful.